Depends on the extent of nationalism. First it's important to establish what nationalism is and then to go on to look at the effects it has.
The nation refers to a group of people who consider themselves to carry a common identity. What that identity truly is is a matter of discussion and broad interpretation; usually this will be some form of genealogical notion that your ancestors have come from the same place for many generations and this builds a sense of unity between the group. This common identity will tend to also be expressed through culture (for example the idea that all Americans love going to watch Nascar and vote at elections. Ok, a very bad example but you get what I mean ...), religion (for example the BNP toting that Britain is a "Christian nation"), language. So nationalism refers to placing a degree of priority towards those that you perceive to share that common identity, whatever that may be. Belgium is notable a country made up of two very distinct nations: the Flemish community and the French community. Similarly it can be said that Canada is a state of two/multiple nations, as with the UK. Whereas less diverse states such as Iceland could be said to be a state of one nationality. This is also further complicated by globalisation and the ease with which people can, today, immigrate to other nations.
Nationalism differs from patriotism because patriotism is a love of a country, nationalism is a love of a people.
In international politics you'll hear the phrase thrown around of "nation state"; the idea of a nation state is a relatively new one with the concept first developing from the French Revolution and the idea that it is the people, the nation, that make the state what it is rather than either something intrinsic to the land or the leader/monarch. Coupled with propaganda during the second World War claiming that the fight was against enslavement of the evil axis, to allow people to continue self determining was what led to the downfall of the European Empires (which the nations that supported the allies as a part of their empires during the war got home and said "well if we were fighting for their freedom and right to be self-determining, what about ours?")
So, is nationalism bad? For a long time I believed intrinsically yes, and to this day I still believe that the only true common identity that we do realistically hold and in this increasingly shrinking world SHOULD hold is that of being fellow human beings. However the answer is much more complicated than a clear cut yes or no.
Nationalism can be good:
It can help to strengthen the sense of unity and belonging that a society feels towards it's fellow inhabitants.
It can help to bring resolve for inhabitants to achieve a distinction of governance and self determination that's not allowed by a monarch, autocrat or dictator (France, perhaps also the fall of the Iron Curtain in '89 and perhaps also the raising of the Iron Curtain in '17)
It can help to bring a people together against an oppressive body that ignores or mistreats the populace of a nation (id est, post-colonial african and asian states [though it's debatable how well this was pulled off, especially in some places], the relative success story that is Ireland).
Nationalism can be bad:
It breeds distrust between people who consider themselves to be of different nationalities (the BNP fear mongering islamophobic platform).
It's used to justify wholesale slaughter of those arrogantly considered to be of a lesser nationality (holocaust, Rwanda).
It can be misplaced, backwards and pointlessly ineffective (Cornwall separatists)
So in answer. Nationalism can be good and bad, it depends on the manner in which it manifests and also the level of zeal with which it does so. To the extent that it can strengthen communal bonds that is by no means a bad thing, but if it rages ahead and creates a sense of arrogance over those of other nationalities then it goes to far and becomes a negative trait.
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Vita brevis,
Occasio praeceps
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